System and method for improving I/O performance by introducing extent pool level I/O credits and user I/O credits throttling on Mapped RAID

ABSTRACT

A method, computer program product, and computer system for receiving, by a computing device, an I/O request for data. A number of storage devices of a plurality of storage devices in a Mapped RAID group that will be used to process the I/O request may be determined. It may be determined that each storage device of the number of storage devices in the Mapped RAID group that will be used to process the I/O request lacks a respective threshold number of credits to process the I/O request. It may be determined whether a cache associated with the Mapped RAID group allows a user I/O queue. If the cache allows the user I/O queue, a user I/O may be placed in the user I/O queue. If the cache does not allow the user I/O queue, the I/O request may be failed.

BACKGROUND

Generally, with the increasing amounts of information being stored, itmay be beneficial to efficiently store and manage that information.While there may be numerous techniques for storing and managinginformation, each technique may have tradeoffs between reliability andefficiency.

BRIEF SUMMARY OF DISCLOSURE

In one example implementation, a method, performed by one or morecomputing devices, may include but is not limited to receiving, by acomputing device, an I/O request for data. A number of storage devicesof a plurality of storage devices in a Mapped RAID group that will beused to process the I/O request may be determined. It may be determinedthat each storage device of the number of storage devices in the MappedRAID group that will be used to process the I/O request lacks arespective threshold number of credits to process the I/O request. Itmay be determined whether a cache associated with the Mapped RAID groupallows a user I/O queue. If the cache allows the user I/O queue, a userI/O may be placed in the user I/O queue. If the cache does not allow theuser I/O queue, the I/O request may be failed.

One or more of the following example features may be included. Aninitial storage device credit for a first type of storage device of theplurality of storage devices may be X. The first type of storage deviceof the plurality of storage devices may be a hard disk drive. An initialstorage device credit for a second type of storage device of theplurality of storage devices may be Y. The second type of storage deviceof the plurality of storage devices may be a flash drive. The thresholdnumber of credits may include at least one of a number of user I/Ocredits in an extent pool and a number of background I/O credits in theextent pool. One or more I/O credits used to process the I/O request maybe returned based upon, at least in part, processing the I/O request.

In another example implementation, a computing system may include one ormore processors and one or more memories configured to performoperations that may include but are not limited to receiving an I/Orequest for data. A number of storage devices of a plurality of storagedevices in a Mapped RAID group that will be used to process the I/Orequest may be determined. It may be determined that each storage deviceof the number of storage devices in the Mapped RAID group that will beused to process the I/O request lacks a respective threshold number ofcredits to process the I/O request. It may be determined whether a cacheassociated with the Mapped RAID group allows a user I/O queue. If thecache allows the user I/O queue, a user I/O may be placed in the userI/O queue. If the cache does not allow the user I/O queue, the I/Orequest may be failed.

One or more of the following example features may be included. Aninitial storage device credit for a first type of storage device of theplurality of storage devices may be X. The first type of storage deviceof the plurality of storage devices may be a hard disk drive. An initialstorage device credit for a second type of storage device of theplurality of storage devices may be Y. The second type of storage deviceof the plurality of storage devices may be a flash drive. The thresholdnumber of credits may include at least one of a number of user I/Ocredits in an extent pool and a number of background I/O credits in theextent pool. One or more I/O credits used to process the I/O request maybe returned based upon, at least in part, processing the I/O request.

In another example implementation, a computer program product may resideon a computer readable storage medium having a plurality of instructionsstored thereon which, when executed across one or more processors, maycause at least a portion of the one or more processors to performoperations that may include but are not limited to receiving an I/Orequest for data. A number of storage devices of a plurality of storagedevices in a Mapped RAID group that will be used to process the I/Orequest may be determined. It may be determined that each storage deviceof the number of storage devices in the Mapped RAID group that will beused to process the I/O request lacks a respective threshold number ofcredits to process the I/O request. It may be determined whether a cacheassociated with the Mapped RAID group allows a user I/O queue. If thecache allows the user I/O queue, a user I/O may be placed in the userI/O queue. If the cache does not allow the user I/O queue, the I/Orequest may be failed.

One or more of the following example features may be included. Aninitial storage device credit for a first type of storage device of theplurality of storage devices may be X. The first type of storage deviceof the plurality of storage devices may be a hard disk drive. An initialstorage device credit for a second type of storage device of theplurality of storage devices may be Y. The second type of storage deviceof the plurality of storage devices may be a flash drive. The thresholdnumber of credits may include at least one of a number of user I/Ocredits in an extent pool and a number of background I/O credits in theextent pool. One or more I/O credits used to process the I/O request maybe returned based upon, at least in part, processing the I/O request.

The details of one or more example implementations are set forth in theaccompanying drawings and the description below. Other possible examplefeatures and/or possible example advantages will become apparent fromthe description, the drawings, and the claims. Some implementations maynot have those possible example features and/or possible exampleadvantages, and such possible example features and/or possible exampleadvantages may not necessarily be required of some implementations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example diagrammatic view of a credit process coupled to anexample distributed computing network according to one or more exampleimplementations of the disclosure;

FIG. 2 is an example diagrammatic view of a computer of FIG. 1 accordingto one or more example implementations of the disclosure;

FIG. 3 is an example diagrammatic view of a storage target of FIG. 1according to one or more example implementations of the disclosure;

FIG. 4 is an example diagrammatic view of a RAID layout of FIG. 1according to one or more example implementations of the disclosure;

FIG. 5 is an example diagrammatic view of a Mapped RAID extent layout ofFIG. 1 according to one or more example implementations of thedisclosure;

FIG. 6 is an example diagrammatic view of an example topology of MappedRAID and Extent Pool according to one or more example implementations ofthe disclosure;

FIG. 7 is an example diagrammatic view of an example Mapped RAIDposition in an I/O stack and cache page starvation according to one ormore example implementations of the disclosure;

FIG. 8 is an example diagrammatic view of an example Mapped RAID groupaccording to one or more example implementations of the disclosure;

FIG. 9 is an example diagrammatic view of an example RAID extentaccording to one or more example implementations of the disclosure;

FIG. 10 is an example flowchart of a credit process according to one ormore example implementations of the disclosure;

FIG. 11 is an example diagrammatic view of an example credits mechanismaccording to one or more example implementations of the disclosure; and

FIG. 12 is an example diagrammatic view of an example I/O layoutaccording to one or more example implementations of the disclosure.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

System Overview:

In some implementations, the present disclosure may be embodied as amethod, system, or computer program product. Accordingly, in someimplementations, the present disclosure may take the form of an entirelyhardware implementation, an entirely software implementation (includingfirmware, resident software, micro-code, etc.) or an implementationcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore, insome implementations, the present disclosure may take the form of acomputer program product on a computer-usable storage medium havingcomputer-usable program code embodied in the medium.

In some implementations, any suitable computer usable or computerreadable medium (or media) may be utilized. The computer readable mediummay be a computer readable signal medium or a computer readable storagemedium. The computer-usable, or computer-readable, storage medium(including a storage device associated with a computing device or clientelectronic device) may be, for example, but is not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, device, or any suitable combination ofthe foregoing. More specific examples (a non-exhaustive list) of thecomputer-readable medium may include the following: an electricalconnection having one or more wires, a portable computer diskette, ahard disk, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), anoptical fiber, a portable compact disc read-only memory (CD-ROM), anoptical storage device, a digital versatile disk (DVD), a static randomaccess memory (SRAM), a memory stick, a floppy disk, a mechanicallyencoded device such as punch-cards or raised structures in a groovehaving instructions recorded thereon, a media such as those supportingthe internet or an intranet, or a magnetic storage device. Note that thecomputer-usable or computer-readable medium could even be a suitablemedium upon which the program is stored, scanned, compiled, interpreted,or otherwise processed in a suitable manner, if necessary, and thenstored in a computer memory. In the context of the present disclosure, acomputer-usable or computer-readable, storage medium may be any tangiblemedium that can contain or store a program for use by or in connectionwith the instruction execution system, apparatus, or device.

In some implementations, a computer readable signal medium may include apropagated data signal with computer readable program code embodiedtherein, for example, in baseband or as part of a carrier wave. In someimplementations, such a propagated signal may take any of a variety offorms, including, but not limited to, electro-magnetic, optical, or anysuitable combination thereof. In some implementations, the computerreadable program code may be transmitted using any appropriate medium,including but not limited to the internet, wireline, optical fibercable, RF, etc. In some implementations, a computer readable signalmedium may be any computer readable medium that is not a computerreadable storage medium and that can communicate, propagate, ortransport a program for use by or in connection with an instructionexecution system, apparatus, or device.

In some implementations, computer program code for carrying outoperations of the present disclosure may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java®, Smalltalk, C++ or the like.Java® and all Java-based trademarks and logos are trademarks orregistered trademarks of Oracle and/or its affiliates. However, thecomputer program code for carrying out operations of the presentdisclosure may also be written in conventional procedural programminglanguages, such as the “C” programming language, PASCAL, or similarprogramming languages, as well as in scripting languages such asJavascript, PERL, or Python. The program code may execute entirely onthe user's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough a local area network (LAN) or a wide area network (WAN), or theconnection may be made to an external computer (for example, through theinternet using an Internet Service Provider). In some implementations,electronic circuitry including, for example, programmable logiccircuitry, field-programmable gate arrays (FPGAs) or other hardwareaccelerators, micro-controller units (MCUs), or programmable logicarrays (PLAs) may execute the computer readable programinstructions/code by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present disclosure.

In some implementations, the flowchart and block diagrams in the figuresillustrate the architecture, functionality, and operation of possibleimplementations of apparatus (systems), methods and computer programproducts according to various implementations of the present disclosure.Each block in the flowchart and/or block diagrams, and combinations ofblocks in the flowchart and/or block diagrams, may represent a module,segment, or portion of code, which comprises one or more executablecomputer program instructions for implementing the specified logicalfunction(s)/act(s). These computer program instructions may be providedto a processor of a general purpose computer, special purpose computer,or other programmable data processing apparatus to produce a machine,such that the computer program instructions, which may execute via theprocessor of the computer or other programmable data processingapparatus, create the ability to implement one or more of thefunctions/acts specified in the flowchart and/or block diagram block orblocks or combinations thereof. It should be noted that, in someimplementations, the functions noted in the block(s) may occur out ofthe order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved.

In some implementations, these computer program instructions may also bestored in a computer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks or combinations thereof.

In some implementations, the computer program instructions may also beloaded onto a computer or other programmable data processing apparatusto cause a series of operational steps to be performed (not necessarilyin a particular order) on the computer or other programmable apparatusto produce a computer implemented process such that the instructionswhich execute on the computer or other programmable apparatus providesteps for implementing the functions/acts (not necessarily in aparticular order) specified in the flowchart and/or block diagram blockor blocks or combinations thereof.

Referring now to the example implementation of FIG. 1, there is showncredit process 10 that may reside on and may be executed by a computer(e.g., computer 12), which may be connected to a network (e.g., network14) (e.g., the internet or a local area network). Examples of computer12 (and/or one or more of the client electronic devices noted below) mayinclude, but are not limited to, a storage system (e.g., a NetworkAttached Storage (NAS) system, a Storage Area Network (SAN)), a personalcomputer(s), a laptop computer(s), mobile computing device(s), a servercomputer, a series of server computers, a mainframe computer(s), or acomputing cloud(s). As is known in the art, a SAN may include one ormore of the client electronic devices, including a RAID device and a NASsystem. In some implementations, each of the aforementioned may begenerally described as a computing device. In certain implementations, acomputing device may be a physical or virtual device. In manyimplementations, a computing device may be any device capable ofperforming operations, such as a dedicated processor, a portion of aprocessor, a virtual processor, a portion of a virtual processor,portion of a virtual device, or a virtual device. In someimplementations, a processor may be a physical processor or a virtualprocessor. In some implementations, a virtual processor may correspondto one or more parts of one or more physical processors. In someimplementations, the instructions/logic may be distributed and executedacross one or more processors, virtual or physical, to execute theinstructions/logic. Computer 12 may execute an operating system, forexample, but not limited to, Microsoft® Windows®; Mac® OS X®; Red Hat®Linux®, Windows® Mobile, Chrome OS, Blackberry OS, Fire OS, or a customoperating system. (Microsoft and Windows are registered trademarks ofMicrosoft Corporation in the United States, other countries or both; Macand OS X are registered trademarks of Apple Inc. in the United States,other countries or both; Red Hat is a registered trademark of Red HatCorporation in the United States, other countries or both; and Linux isa registered trademark of Linus Torvalds in the United States, othercountries or both).

In some implementations, as will be discussed below in greater detail, acredit process, such as credit process 10 of FIG. 1, may receive, by acomputing device, an I/O request (e.g., I/O request 15) for data. Anumber of storage devices of a plurality of storage devices in a MappedRAID group that will be used to process the I/O request may bedetermined. It may be determined whether each storage device of thenumber of storage devices in the Mapped RAID group that will be used toprocess the I/O request has a respective threshold number of credits toprocess the I/O request. If each storage device of the number of storagedevices in the Mapped RAID group that will be used to process the I/Orequest has the respective threshold number of credits, the I/O requestmay be processed. If at least one storage device of the number ofstorage devices in the Mapped RAID group that will be used to processthe I/O request lacks the respective threshold number of credits, theI/O request may be queued.

In some implementations, the instruction sets and subroutines of creditprocess 10, which may be stored on storage device, such as storagedevice 16, coupled to computer 12, may be executed by one or moreprocessors and one or more memory architectures included within computer12. In some implementations, storage device 16 may include but is notlimited to: a hard disk drive; all forms of flash memory storagedevices; a tape drive; an optical drive; a RAID array (or other array);a random access memory (RAM); a read-only memory (ROM); or combinationthereof. In some implementations, storage device 16 may be organized asan extent, an extent pool, a RAID extent (e.g., an example 4D+1P R5,where the RAID extent may include, e.g., five storage device extentsthat may be allocated from, e.g., five different storage devices), amapped RAID (e.g., a collection of RAID extents), or combinationthereof.

In some implementations, network 14 may be connected to one or moresecondary networks (e.g., network 18), examples of which may include butare not limited to: a local area network; a wide area network; or anintranet, for example.

In some implementations, computer 12 may include a data store, such as adatabase (e.g., relational database, object-oriented database,triplestore database, etc.) and may be located within any suitablememory location, such as storage device 16 coupled to computer 12. Insome implementations, data, metadata, information, etc. describedthroughout the present disclosure may be stored in the data store. Insome implementations, computer 12 may utilize any known databasemanagement system such as, but not limited to, DB2, in order to providemulti-user access to one or more databases, such as the above notedrelational database. In some implementations, the data store may also bea custom database, such as, for example, a flat file database or an XMLdatabase. In some implementations, any other form(s) of a data storagestructure and/or organization may also be used. In some implementations,credit process 10 may be a component of the data store, a standaloneapplication that interfaces with the above noted data store and/or anapplet/application that is accessed via client applications 22, 24, 26,28. In some implementations, the above noted data store may be, in wholeor in part, distributed in a cloud computing topology. In this way,computer 12 and storage device 16 may refer to multiple devices, whichmay also be distributed throughout the network. An example cloudcomputing environment that may be used with the disclosure may includebut is not limited to, e.g., Elastic Cloud Storage (ECS™) from Dell EMC™of Hopkinton, Mass. In some implementations, other cloud computingenvironments may be used without departing from the scope of thedisclosure.

In some implementations, computer 12 may execute a storage managementapplication (e.g., storage management application 21), examples of whichmay include, but are not limited to, e.g., a storage system application,a cloud computing application, a data synchronization application, adata migration application, a garbage collection application, or otherapplication that allows for the implementation and/or management of datain a clustered (or non-clustered) environment (or the like). In someimplementations, credit process 10 and/or storage management application21 may be accessed via one or more of client applications 22, 24, 26,28. In some implementations, credit process 10 may be a standaloneapplication, or may be an applet/application/script/extension that mayinteract with and/or be executed within storage management application21, a component of storage management application 21, and/or one or moreof client applications 22, 24, 26, 28. In some implementations, storagemanagement application 21 may be a standalone application, or may be anapplet/application/script/extension that may interact with and/or beexecuted within credit process 10, a component of credit process 10,and/or one or more of client applications 22, 24, 26, 28. In someimplementations, one or more of client applications 22, 24, 26, 28 maybe a standalone application, or may be anapplet/application/script/extension that may interact with and/or beexecuted within and/or be a component of credit process 10 and/orstorage management application 21. Examples of client applications 22,24, 26, 28 may include, but are not limited to, e.g., a storage systemapplication, a cloud computing application, a data synchronizationapplication, a data migration application, a garbage collectionapplication, or other application that allows for the implementationand/or management of data in a clustered (or non-clustered) environment(or the like), a standard and/or mobile web browser, an emailapplication (e.g., an email client application), a textual and/or agraphical user interface, a customized web browser, a plugin, anApplication Programming Interface (API), or a custom application. Theinstruction sets and subroutines of client applications 22, 24, 26, 28,which may be stored on storage devices 30, 32, 34, 36, coupled to clientelectronic devices 38, 40, 42, 44, may be executed by one or moreprocessors and one or more memory architectures incorporated into clientelectronic devices 38, 40, 42, 44.

In some implementations, one or more of storage devices 30, 32, 34, 36,may include but are not limited to: hard disk drives; flash drives, tapedrives; optical drives; RAID arrays; random access memories (RAM); andread-only memories (ROM). Examples of client electronic devices 38, 40,42, 44 (and/or computer 12) may include, but are not limited to, apersonal computer (e.g., client electronic device 38), a laptop computer(e.g., client electronic device 40), a smart/data-enabled, cellularphone (e.g., client electronic device 42), a notebook computer (e.g.,client electronic device 44), a tablet, a server, a television, a smarttelevision, a media (e.g., video, photo, etc.) capturing device, and adedicated network device. Client electronic devices 38, 40, 42, 44 mayeach execute an operating system, examples of which may include but arenot limited to, Android″, Apple® iOS®, Mac® OS X®; Red Hat® Linux®,Windows® Mobile, Chrome OS, Blackberry OS, Fire OS, or a customoperating system.

In some implementations, one or more of client applications 22, 24, 26,28 may be configured to effectuate some or all of the functionality ofcredit process 10 (and vice versa). Accordingly, in someimplementations, credit process 10 may be a purely server-sideapplication, a purely client-side application, or a hybridserver-side/client-side application that is cooperatively executed byone or more of client applications 22, 24, 26, 28 and/or credit process10.

In some implementations, one or more of client applications 22, 24, 26,28 may be configured to effectuate some or all of the functionality ofstorage management application 21 (and vice versa). Accordingly, in someimplementations, storage management application 21 may be a purelyserver-side application, a purely client-side application, or a hybridserver-side/client-side application that is cooperatively executed byone or more of client applications 22, 24, 26, 28 and/or storagemanagement application 21. As one or more of client applications 22, 24,26, 28, credit process 10, and storage management application 21, takensingly or in any combination, may effectuate some or all of the samefunctionality, any description of effectuating such functionality viaone or more of client applications 22, 24, 26, 28, credit process 10,storage management application 21, or combination thereof, and anydescribed interaction(s) between one or more of client applications 22,24, 26, 28, credit process 10, storage management application 21, orcombination thereof to effectuate such functionality, should be taken asan example only and not to limit the scope of the disclosure.

In some implementations, one or more of users 46, 48, 50, 52 may accesscomputer 12 and credit process 10 (e.g., using one or more of clientelectronic devices 38, 40, 42, 44) directly through network 14 orthrough secondary network 18. Further, computer 12 may be connected tonetwork 14 through secondary network 18, as illustrated with phantomlink line 54. Credit process 10 may include one or more user interfaces,such as browsers and textual or graphical user interfaces, through whichusers 46, 48, 50, 52 may access credit process 10.

In some implementations, the various client electronic devices may bedirectly or indirectly coupled to network 14 (or network 18). Forexample, client electronic device 38 is shown directly coupled tonetwork 14 via a hardwired network connection. Further, clientelectronic device 44 is shown directly coupled to network 18 via ahardwired network connection. Client electronic device 40 is shownwirelessly coupled to network 14 via wireless communication channel 56established between client electronic device 40 and wireless accesspoint (i.e., WAP) 58, which is shown directly coupled to network 14. WAP58 may be, for example, an IEEE 802.11a, 802.11b, 802.11g, 802.11n,802.11ac, Wi-Fi®, RFID, and/or Bluetooth™ (including Bluetooth™ LowEnergy) device that is capable of establishing wireless communicationchannel 56 between client electronic device 40 and WAP 58. Clientelectronic device 42 is shown wirelessly coupled to network 14 viawireless communication channel 60 established between client electronicdevice 42 and cellular network/bridge 62, which is shown by exampledirectly coupled to network 14.

In some implementations, some or all of the IEEE 802.11x specificationsmay use Ethernet protocol and carrier sense multiple access withcollision avoidance (i.e., CSMA/CA) for path sharing. The various802.11x specifications may use phase-shift keying (i.e., PSK) modulationor complementary code keying (i.e., CCK) modulation, for example.Bluetooth™ (including Bluetooth™ Low Energy) is a telecommunicationsindustry specification that allows, e.g., mobile phones, computers,smart phones, and other electronic devices to be interconnected using ashort-range wireless connection. Other forms of interconnection (e.g.,Near Field Communication (NFC)) may also be used.

In some implementations, various I/O requests (e.g., I/O request 15) maybe sent from, e.g., client applications 22, 24, 26, 28 to, e.g.,computer 12. Examples of I/O request 15 may include but are not limitedto, data write requests (e.g., a request that content be written tocomputer 12) and data read requests (e.g., a request that content beread from computer 12).

Data Storage System:

Referring also to the example implementation of FIGS. 2-3 (e.g., wherecomputer 12 may be configured as a data storage system), computer 12 mayinclude storage processor 100 and a plurality of storage targets (e.g.,storage targets 102, 104, 106, 108, 110). In some implementations,storage targets 102, 104, 106, 108, 110 may include any of theabove-noted storage devices. In some implementations, storage targets102, 104, 106, 108, 110 may be configured to provide various levels ofperformance and/or high availability. For example, storage targets 102,104, 106, 108, 110 may be configured to form a non-fully-duplicativefault-tolerant data storage system (such as a non-fully-duplicative RAIDdata storage system), examples of which may include but are not limitedto: RAID 3 arrays, RAID 4 arrays, RAID 5 arrays, and/or RAID 6 arrays.It will be appreciated that various other types of RAID arrays may beused without departing from the scope of the present disclosure.

While in this particular example, computer 12 is shown to include fivestorage targets (e.g., storage targets 102, 104, 106, 108, 110), this isfor example purposes only and is not intended limit the presentdisclosure. For instance, the actual number of storage targets may beincreased or decreased depending upon, e.g., the level ofredundancy/performance/capacity required.

Further, the storage targets (e.g., storage targets 102, 104, 106, 108,110) included with computer 12 may be configured to form a plurality ofdiscrete storage arrays. For instance, and assuming for example purposesonly that computer 12 includes, e.g., ten discrete storage targets, afirst five targets (of the ten storage targets) may be configured toform a first RAID array and a second five targets (of the ten storagetargets) may be configured to form a second RAID array.

In some implementations, one or more of storage targets 102, 104, 106,108, 110 may be configured to store coded data (e.g., via storagemanagement application 21), wherein such coded data may allow for theregeneration of data lost/corrupted on one or more of storage targets102, 104, 106, 108, 110. Examples of such coded data may include but isnot limited to parity data and Reed-Solomon data. Such coded data may bedistributed across all of storage targets 102, 104, 106, 108, 110 or maybe stored within a specific storage target.

Examples of storage targets 102, 104, 106, 108, 110 may include one ormore data arrays, wherein a combination of storage targets 102, 104,106, 108, 110 (and any processing/control systems associated withstorage management application 21) may form data array 112.

The manner in which computer 12 is implemented may vary depending upone.g., the level of redundancy/performance/capacity required. Forexample, computer 12 may be configured as a SAN (i.e., a Storage AreaNetwork), in which storage processor 100 may be, e.g., a dedicatedcomputing system and each of storage targets 102, 104, 106, 108, 110 maybe a RAID device.

In the example where computer 12 is configured as a SAN, the variouscomponents of computer 12 (e.g., storage processor 100, and storagetargets 102, 104, 106, 108, 110) may be coupled using networkinfrastructure 114, examples of which may include but are not limited toan Ethernet (e.g., Layer 2 or Layer 3) network, a fiber channel network,an InfiniBand network, or any other circuit switched/packet switchednetwork.

As discussed above, various I/O requests (e.g., I/O request 15) may begenerated. For example, these I/O requests may be sent from, e.g.,client applications 22, 24, 26, 28 to, e.g., computer 12.Additionally/alternatively (e.g., when storage processor 100 isconfigured as an application server or otherwise), these I/O requestsmay be internally generated within storage processor 100 (e.g., viastorage management application 21). Examples of I/O request 15 mayinclude but are not limited to data write request 116 (e.g., a requestthat content 118 be written to computer 12) and data read request 120(e.g., a request that content 118 be read from computer 12).

In some implementations, during operation of storage processor 100,content 118 to be written to computer 12 may be received and/orprocessed by storage processor 100 (e.g., via storage managementapplication 21). Additionally/alternatively (e.g., when storageprocessor 100 is configured as an application server or otherwise),content 118 to be written to computer 12 may be internally generated bystorage processor 100 (e.g., via storage management application 21).

As discussed above, the instruction sets and subroutines of storagemanagement application 21, which may be stored on storage device 16included within computer 12, may be executed by one or more processorsand one or more memory architectures included with computer 12.Accordingly, in addition to being executed on storage processor 100,some or all of the instruction sets and subroutines of storagemanagement application 21 (and/or credit process 10) may be executed byone or more processors and one or more memory architectures includedwith data array 112.

In some implementations, storage processor 100 may include front endcache memory system 122. Examples of front end cache memory system 122may include but are not limited to a volatile, solid-state, cache memorysystem (e.g., a dynamic RAM cache memory system), a non-volatile,solid-state, cache memory system (e.g., a flash-based, cache memorysystem), and/or any of the above-noted storage devices.

In some implementations, storage processor 100 may initially storecontent 118 within front end cache memory system 122. Depending upon themanner in which front end cache memory system 122 is configured, storageprocessor 100 (e.g., via storage management application 21) mayimmediately write content 118 to data array 112 (e.g., if front endcache memory system 122 is configured as a write-through cache) or maysubsequently write content 118 to data array 112 (e.g., if front endcache memory system 122 is configured as a write-back cache).

In some implementations, one or more of storage targets 102, 104, 106,108, 110 may include a backend cache memory system. Examples of thebackend cache memory system may include but are not limited to avolatile, solid-state, cache memory system (e.g., a dynamic RAM cachememory system), a non-volatile, solid-state, cache memory system (e.g.,a flash-based, cache memory system), and/or any of the above-notedstorage devices.

Storage Targets:

As discussed above, one or more of storage targets 102, 104, 106, 108,110 may be a RAID device. For instance, and referring also to FIG. 3,there is shown example target 150, wherein target 150 may be one exampleimplementation of a RAID implementation of, e.g., storage target 102,storage target 104, storage target 106, storage target 108, and/orstorage target 110. Examples of storage devices 154, 156, 158, 160, 162may include one or more electro-mechanical hard disk drives, one or moresolid-state/flash devices, and/or any of the above-noted storagedevices.

In some implementations, target 150 may include storage processor 152and a plurality of storage devices (e.g., storage devices 154, 156, 158,160, 162). Storage devices 154, 156, 158, 160, 162 may be configured toprovide various levels of performance and/or high availability (e.g.,via storage management application 21). For example, one or more ofstorage devices 154, 156, 158, 160, 162 (or any of the above-notedstorage devices) may be configured as a RAID 0 array, in which data isstriped across storage devices. By striping data across a plurality ofstorage devices, improved performance may be realized. However, RAID 0arrays may not provide a level of high availability. Accordingly, one ormore of storage devices 154, 156, 158, 160, 162 (or any of theabove-noted storage devices) may be configured as a RAID 1 array, inwhich data is mirrored between storage devices. By mirroring databetween storage devices, a level of high availability may be achieved asmultiple copies of the data may be stored within storage devices 154,156, 158, 160, 162.

While storage devices 154, 156, 158, 160, 162 are discussed above asbeing configured in a RAID 0 or RAID 1 array, this is for examplepurposes only and not intended to limit the present disclosure, as otherconfigurations are possible. For example, storage devices 154, 156, 158,160, 162 may be configured as a RAID 3, RAID 4, RAID 5 or RAID 6 array.

While in this particular example, target 150 is shown to include fivestorage devices (e.g., storage devices 154, 156, 158, 160, 162), this isfor example purposes only and not intended to limit the presentdisclosure. For instance, the actual number of storage devices may beincreased or decreased depending upon, e.g., the level ofredundancy/performance/capacity required.

In some implementations, one or more of storage devices 154, 156, 158,160, 162 may be configured to store (e.g., via storage managementapplication 21) coded data, wherein such coded data may allow for theregeneration of data lost/corrupted on one or more of storage devices154, 156, 158, 160, 162. Examples of such coded data may include but arenot limited to parity data and Reed-Solomon data. Such coded data may bedistributed across all of storage devices 154, 156, 158, 160, 162 or maybe stored within a specific storage device.

The manner in which target 150 is implemented may vary depending upone.g., the level of redundancy/performance/capacity required. Forexample, target 150 may be a RAID device in which storage processor 152is a RAID controller card and storage devices 154, 156, 158, 160, 162are individual “hot-swappable” hard disk drives. Another example oftarget 150 may be a RAID system, examples of which may include but arenot limited to an NAS (i.e., Network Attached Storage) device or a SAN(i.e., Storage Area Network).

In some implementations, storage target 150 may execute all or a portionof storage management application 21. The instruction sets andsubroutines of storage management application 21, which may be stored ona storage device (e.g., storage device 164) coupled to storage processor152, may be executed by one or more processors and one or more memoryarchitectures included with storage processor 152. Storage device 164may include but is not limited to any of the above-noted storagedevices.

As discussed above, computer 12 may be configured as a SAN, whereinstorage processor 100 may be a dedicated computing system and each ofstorage targets 102, 104, 106, 108, 110 may be a RAID device.Accordingly, when storage processor 100 processes data requests 116,120, storage processor 100 (e.g., via storage management application 21)may provide the appropriate requests/content (e.g., write request 166,content 168 and read request 170) to, e.g., storage target 150 (which isrepresentative of storage targets 102, 104, 106, 108 and/or 110).

In some implementations, during operation of storage processor 152,content 168 to be written to target 150 may be processed by storageprocessor 152 (e.g., via storage management application 21). Storageprocessor 152 may include cache memory system 172. Examples of cachememory system 172 may include but are not limited to a volatile,solid-state, cache memory system (e.g., a dynamic RAM cache memorysystem) and/or a non-volatile, solid-state, cache memory system (e.g., aflash-based, cache memory system). During operation of storage processor152, content 168 to be written to target 150 may be received by storageprocessor 152 (e.g., via storage management application 21) andinitially stored (e.g., via storage management application 21) withinfront end cache memory system 172.

Example RAID Group:

As discussed above, and referring at least to the example implementationof FIG. 4, an example 4D+1P RAID 5 layout 400 a that may be managed(e.g., via storage management application 21) is shown. In the example,data may be distributed across the storage devices (e.g., drives) in oneof several ways, referred to as RAID levels, depending on the requiredlevel of redundancy and performance. As noted above, while one or moreof the figures may shows disks as the storage device, it will beappreciated that any of the storage devices discussed throughout may beused.

Shown for example purposes only, RAID 5 may consist of block levelstriping with distributed parity. Parity information may be distributedamong the drives. In the above example, each stripe may consist of fiveblocks, which may include four data blocks (e.g., D0, D1, D2, D3) andone parity block (e.g., P). Upon failure of a single drive, subsequentreads may be calculated from the distributed parity such that no data islost. At the same time, a “hot spare” storage device may be selected toreplace the dead storage device, and all the data on the failed drivemay be rebuilt and written to the new drive. For instance, and referringat least to the example implementation of FIG. 4, an example RAID 5rebuild 400 b of the example 4D+1P RAID 5 layout of 400 a is shown.

As storage device capacity increases, the rebuild time may alsoincrease. As a result, there may be an increased risk of a doublestorage device failure, which may lead to data loss. It will beappreciated that the rebuild time may be subject to the write bandwidthof the hot spare storage device, which may become a bottleneck. In somesituations, it may be difficult to reduce the rebuild time for RAID. Insome implementations, Mapped RAID technology have help resolve thisissue.

Example Mapped RAID Group:

In some implementations, and referring at least to the exampleimplementation of FIG. 5, an example Mapped RAID extent layout 500 amanaged (e.g., via storage management application 21) is shown (e.g., a4D+1P RAID 5 over N disks, where N is greater than 5). Generally, MappedRAID may be created on top of a disk (or other storage device) pool,which may include N disks (or other storage devices). Broadly speaking,each disk may be viewed as a set of continuous, non-overlapping, fixedsized disk extents. In the example of FIG. 5, while creating a RAIDextent, 5 disk extents may be selected from 5 different disks. Some diskextents on each disk may be reserved as hot spare disk extents (e.g.,instead of reserving the whole disk as a hot spare disk, which may betypical with a traditional RAID group).

Generally, it may be expected that storage management application 21 mayevenly distribute the RAID extents to all disks in the pool, and furtherexpected that all disk space is consumed no matter whether theircapacity is the same or not. Typically, when one disk fails, thereplacement may be found from other live disks for each disk extent onthe dead drive. For instance, and referring at least to the exampleimplementation of FIG. 5, an example disk extent replacement during diskfail in Mapped RAID layout 500 b of the example Mapped RAID extentlayout 500 a is shown.

Generally, an example limitation for the disk extents selection may bethat there should be a guarantee that each RAID extent straddles 5different disks (e.g., assuming the layout in FIG. 5). Moreover, storagemanagement application 21 may be expected to distribute the dead diskextent replacements evenly to live disks.

Example RAID Extent Layout

In some implementations, one Mapped RAID group may be a set of orderedRAID extents. When creating a Mapped RAID group, storage managementapplication 21 may need to allocate many RAID extents from the extentpool. As noted above, one RAID extent may need a RAID width number ofdisk extents from different disks in the extent pool. Generally, storagemanagement application 21 may allocate the RAID extents one by one. Forease of explanation only, assume the disk number in the pool is N, andthe RAID extent width is M. In the example, there should be C_(n) ^(m)possible ways to combine a RAID extent. A typical case may be forstorage management application 21 to create 4D+1P mapped RAID 5 on topof 16 disks, where there are C₁₆ ⁵=4368 ways to allocate a RAID extenton top of 16 disks. In some implementations, for each RAID extentallocation, storage management application 21 may loop over all thepossible choices, and select the best one. In the example, the searchingdepth may be C_(n) ^(m).

Example Mapped RAID and Extent Pool Topology

Referring at least to the example implementation of FIG. 6, an exampletopology 600 of Mapped RAID and Extent Pool is shown. In someimplementations, the extent pool may support both SSDs and HDDstogether. A physical drive object (PDO) may be used to describe eachdrive, which may include the drive type, sector size, and performancetier, etc. In the example, all drives in the extent pool should have thesimilar characteristic. In the extent pool, each disk may be split intoa set of disk extents, where a number of disk extents may be selectedand composed together and a Mapped RAID object may operate a RAIDalgorithm on the them. Generally, each extent pool may, e.g., (1)include anywhere from a small number of drives to hundreds of drives (ormore or less), (2) there may be more than one Mapped RAID object createdon it, and they may share the same set of drives in the extent pool, and(3) each Mapped RAID object may expose their capacity to a multi-corecache (MCC) through a Flare LUN (FLU).

Mapped RAID Position in the I/O Stack

Referring at least to the example implementation of FIG. 7, an exampleMapped RAID position in an I/O stack 700 a is shown. As noted above,Mapped RAID may be generally created on top of an extent pool, which isa collection of disks (or other storage device type). The space ofMapped RAID may be exposed to a multi-core cache (MCC) through a FlareLUN (FLU). MCC in the storage system may act as a buffer cache. Memoryin the MCC may be organized into memory pages. A memory page may be,e.g., an 8 KB size. From the system perspective, a user I/O may first goto the MCC first. When a read I/O is sent to the MCC, the I/O mayallocate memory pages in the MCC, and then send to the FLU, where MappedRAID may break the I/O down to disk extents according to RAID geometry,and then the extent pool may forward the I/O to downstream disks. Afterthe I/O is returned back, the user data may generally be said to havebeen read into memory pages. Generally, when a write I/O is sent to theMCC, the I/O data may be copied to the MCC's memory pages (or the memorymay be allocated from the memory managed by the MCC, so data copy may beavoided), and then the write I/O may be completed. When some conditionis met e.g., lack of free page or timer triggered, a background flushmay be triggered. The MCC may flush dirty pages to the backend. The MCCmay flush dirty pages sequentially in logical block address (LBA)increasing order. Thus, the MCC flush I/O pattern may be sequentiallyoriented. Generally, the host sends the 8K random write I/O to the MCC,and the MCC may reorganize these random host I/Os to sequentially flushthe I/Os and send down to the FLU. The flush I/O may go through thesoftware stack top and down just like the read I/O. This kind ofbehavior for the MCC's may require that the FLU and Mapped RAID shouldoptimize sequential write I/O performance

In addition to the user I/O, Mapped RAID may also generate internal I/Oswhile handling disk failure in a background service. If one of thedrives fails, the extent pool may replace the broken disk extents withspares and notify the Mapped RAID to reconstruct the user data. In orderto reduce the risk of data loss, Mapped RAID may rebuild several diskextents in parallel. During the rebuild, Mapped RAID may service theuser I/O as well. RAID background services may also generate backgroundI/O.

Referring at least to the example implementation of FIG. 7, an examplecache page starvation on a fast FLU 700 b is shown. In the example, fromthe I/O stack of Mapped RAID, all the user I/Os may be sent to the DRAMCache first. The DRAM Cache space may be split into a bundle of pages.DRAM Cache may need to allocate the pages while processing the user I/O,and generally, these pages cannot be released until the dirty paged getflushed to disk.

In some implementations, the pages may be shared in system. While theuser program accesses several FLUs in parallel, the user I/Os maycompete with each other to get the free pages. Since the drives may havedifferent performance levels, some FLUs may have better I/O performancethan others, and the I/Os sent to the faster FLU may be completedearlier. All the user I/Os sent to the DRAM Cache may be running inparallel. While the slow and fast FLU service I/Os together, since theoutstanding I/Os may be returned back later on the slower FLU, that FLUmay hold the most cache pages. As a result, this may cause the user I/Osof the faster FLU to starve due to insufficient paged resources. Thus,there may be a need to solve this example issue and determine amechanism to let the DRAM Cache work with Mapped RAID together.

Generally, there may be situations where the I/O load may be unbalanced,which may result in disk I/O starvation. For instance, in the extentpool, the I/O load of each drive may be different due to RAID extentlayout and the user I/O pattern. Even if the drives have the sameperformance tier, and the disk extents are evenly distributed to alldrives, the I/O load may still be unbalance among all drives. Forinstance, and referring to FIG. 8, an example and non-limiting MappedRAID group 800 (illustrating disk I/O load during writes) is shown. Inthe example, there are four Mapped RAID groups (e.g., 4D+1P RAID 5 onone extent pool). In the example, assume for example purposes only thatI/Os are issued to each Mapped RAID at the same time, where these I/Oswill write each disk extent in that RAID extent once. It may be possiblethat due to RAID technology, the write I/Os may also update the paritydisk extents. After writes break down to disks through mapping, eachdisk may have different I/O loads. The last disk N may need to servicethe most I/Os and the load of disks 1, 2, 4 are light.

As similarly discussed above regarding a slow FLU, the slowest disk maybecome the bottle neck of the whole extent pool. This may be due to, forexample, (1) the pending I/Os on the slow disks may consume a lot ofsystem resource (e.g., memory, lock, etc.), (2) the I/O response time ofMapped RAID may depend on the slowest drives (e.g., the response timemay increase significantly once the drive is over loaded), and (3) theI/O load on each disk may unbalance, thus some disk(s) may be starveddue to a lack of memory, etc.

Another potential example that may introduce unbalanced disk I/O may beshown in the example RAID extents 900 of FIG. 9. In the example, thereare a plurality (e.g., two) “hot” RAID extents. These two RAID extentsmay have an overlap on disk five, which may mean that both RAID extents(RAID extent 1 and RAID extent 2) allocate one disk extent on disk five.Thus, in this example case, the I/O workload on disk five may be heavierthan other disks. Moreover, if one disk is severely overloaded, the I/Oresponse time on that disk may be dramatically increased. In otherwords, the disk will become “slow”. A slow disk may severely reduce theoverall extent pool I/O performance.

As will be discussed below, credit process 10 may at least help, e.g.,the improvement of an existing storage technology, necessarily rooted incomputer technology in order to overcome an example and non-limitingproblem specifically arising in the realm of data storage. For instance,credit process 10 may use an efficient process to improve extent poolI/O performance on Mapped RAID.

The Credit Process:

As discussed above and referring also at least to the exampleimplementations of FIGS. 10-12, credit process 10 may receive 1000, by acomputing device, an I/O request for data. Credit process 10 maydetermine 1002 a number of storage devices of a plurality of storagedevices in a Mapped RAID group that will be used to process the I/Orequest. Credit process 10 may determine 1004 that each storage deviceof the number of storage devices in the Mapped RAID group that will beused to process the I/O request lacks a respective threshold number ofcredits to process the I/O request. Credit process 10 may determine 1005whether a cache associated with the Mapped RAID group allows a user I/Oqueue. If the cache allows the user I/O queue, credit process 10 mayplace 1006 a user I/O in the user I/O queue. If the cache does not allowthe user I/O queue, credit process 10 may fail 1008 the I/O request.

In some implementations, and referring at least to the exampleimplementation of FIG. 11, an example credits mechanism 1100 is shown.In the example, there is shown a Mapped Raid Group level (e.g., MRG1102), an extent pool level (e.g., Ext Pool 1104), and a Physical DriveObject level (e.g., PDO 1106). Further in the example, Ext Pool 1104level may include User I/O credits and Background I/O credits, and PDO1106 level may include disk I/O credits. As will be discussed below, foreach I/O received, credit process 10 may first take the I/O credits onthe Ext pool 1104 level, and then take the disk I/O credits on the PDO1106 level. The disk I/O credits may help prevent the slow disks of theextent pool from getting overloaded, and may also help avoid slow disksabsorbing too many I/Os that may drag down the whole RAID response time.As noted above, the I/O load of each disk (e.g., HDD or SSD, etc.) maybe different due to the user I/O pattern. The disk with the heavier loadmay need more time to process I/Os; however, the disk with the smallerload may finish processing the I/Os in a shorter period of time. With acontinuous heavy I/O load, more and more I/Os may be waiting to beprocessed on the slower disk(s), and in contrast, the I/Os waiting to beprocessed on the faster disk(s) may become fewer and fewer. Thissituation may remain until the total I/O credits are exhausted.

In some implementations, credit process 10 may receive 1000, by acomputing device, an I/O request for data. For instance, assume forexample purposes only that a user (e.g., user 46) is requesting someform of data request (e.g., read or write). In the example, user 46 (viacomputing device 36) may send an I/O request (e.g., I/O 15) for data(e.g., stored in storage target 150). In the example, I/O 15 may bereceived 1000 by credit process 10.

In some implementations, credit process 10 may determine 1002 a numberof storage devices of a plurality of storage devices in a Mapped RAIDgroup that will be used to process the I/O request. For example, MappedRAID (e.g., via credit process 10) may be responsible for breaking downI/O 15 to some disk I/O, leveraging the RAID algorithm and RAID extentmapping. As will be appreciated, different RAID types may have differentdata layout algorithms. For example, mirror RAID may save one copy ofdata on each disk, but parity RAID may use some disks to save, e.g.,user data and one (e.g., RAID 5) or two (e.g., RAID 6) disks to saveparity data, where the parity data may be rotated among the associateddisks. For Mapped RAID, credit process 10 may leverage both the RAIDalgorithm and the associated saved mapping table, since, e.g., creditprocess 10 may need to have knowledge of where its disk extents for eachRAID extent are allocated and from which disks. Using the Mapped RAIDobject, credit process 10 may be able to break down the received 1000I/O to the disk I/Os, to determine 1002 how many storage devices in theMapped RAID group will be used to process I/O 15.

An example of this will be discussed below for a RAID 5 implementation,however, it will be appreciated that other RAID implementations may alsobe used without departing from the scope of the disclosure. With theexample RAID 5 implementation, and referring at least to the example I/Olayout 1200 of FIG. 12, assume for example purposes only that there is a4+1 RAID 5 set up. In the example, should I/O 15 be a read I/O, creditprocess 10 may determine 1002 that two disks in the Mapped RAID groupwill be involved (i.e., “touched”) for processing the read I/O (e.g.,data disks D1 and D2). As a result, the associated I/O credits in thisexample may be two. As another example, should I/O 15 be a write I/O,credit process 10 may determine 1002 that three disks in the Mapped RAIDgroup will be involved (i.e., “touched”) for processing the write I/O(e.g., data disks D1 and D2, and also a parity disk P since for a writeI/O, credit process 10 may update the parity). As a result, theassociated I/O credit in this example may be three.

Credit process 10 may determine 1004 that each storage device of thenumber of storage devices in the Mapped RAID group that will be used toprocess the I/O request lacks a respective threshold number of creditsto process the I/O request. For instance, in some implementations, thethreshold number of credits may include at least one of a number of userI/O credits in an extent pool and a number of background I/O credits inthe extent pool. For example, for any I/O (including I/O 15), creditprocess 10 may take two kinds of I/O credits (e.g., first taking thecredits on Ext pool 1104 level, where read/write user I/O may take userI/O credits and Background I/O (e.g., rebuild, verify, copy) may takeBackground I/O credits).

In some implementations, when determining the respective thresholdnumber of credits to process the I/O request exists, an initial storagedevice credit for a first type of storage device of the plurality ofstorage devices may be X, where the first type of storage device of theplurality of storage devices may be a hard disk drive, and an initialstorage device credit for a second type of storage device of theplurality of storage devices may be Y, where the second type of storagedevice of the plurality of storage devices may be a flash drive. In theexample, for each disk in the extent pool, credit process 10 mayinitiate the disk credits per the example function below:

For a HDD (e.g., spindle drive), initial credits X may include, e.g.:Initial disk credits=I/O credits per disk*2

For a SDD (e.g., flash drive), initial credits Y may include, e.g.:Initial disk credits=I/O credits per disk*8.

That is, the initial disk credits may be N times the I/O credits perdisks, where for a HDD, N may be two, and for SDD, N may be eight.However, it will be appreciated that N may be more or less withoutdeparting from the scope of the disclosure. For each user I/O, theassociated RAID object (e.g., via credit process 10) may calculate whichdisk position I/O 15 may access and request the disk credits togetherwith I/O credits. In the example, “I/O credits per disk” may representhow many I/Os the disk may process in parallel. In some implementations,such an attribute may be obtained from PDO 1106. Thus, for HDD, creditprocess may set the disk credits to two times the I/O credits per disk,and for SSD, credit process may set the disk credits to eight times theI/O credits per disk.

In some implementations, if each storage device of the number of storagedevices in the Mapped RAID group that will be used to process the I/Orequest has the respective threshold number of credits, credit process10 may process the I/O request. For instance, assume for examplepurposes only that each of the disks in the Mapped RAID group that willbe involved in processing I/O 15 currently has Z disk I/O credits.Further assume that the cost of each disk to process I/O 15 that will beinvolved in processing I/O 15 is Z−1. Thus, in the example, thethreshold number of credits is determined 1004 by credit process 10 foreach disk involved in processing I/O 15 to be Z−1. In the example,because each respective disk has at least the threshold number ofcredits required to process I/O 15 (e.g., Z−1), credit process 10 mayprocess I/O 15.

In some implementations, if at least one storage device of the number ofstorage devices in the Mapped RAID group that will be used to processthe I/O request lacks the respective threshold number of credits, creditprocess 10 may queue the I/O request. For instance, assume for examplepurposes only that D1 in the Mapped RAID group that will be involved inprocessing I/O 15 currently has Z+1 disk I/O credits, D2 in the MappedRAID group that will be involved in processing I/O 15 currently has Z+1disk I/O credits, and that P in the Mapped RAID group that will beinvolved in processing I/O 15 currently has Z disk I/O credits. Furtherassume that the cost of each disk to process I/O 15 that will beinvolved in processing I/O 15 is Z. Thus, in the example, the thresholdnumber of credits is determined 1004 by credit process 10 to be Z. Inthe example, because D1 and D2 each have at least the threshold numberof credits required to process I/O 15 (e.g., Z+1), but because P with Zdisk I/O credits does not have at least the threshold number of creditsrequired to process I/O 15 (e.g., Z+1), at least one disk in the MappedRAID group (i.e., P) that will be involved in processing I/O 15 lacks atleast the threshold number of credits required to process I/O 15,therefore credit process 10 may queue I/O 15. In some implementations,should credit process 10 determine later that the appropriate amount ofcredits is available, credit process 10 may then wake up and process I/O15, and may again queue I/O 15 if there are still not enough credits.

In some implementations, credit process 10 may determine 1005 whether acache associated with the Mapped RAID group allows a user I/O queue. Ifthe cache allows the user I/O queue, credit process 10 may place 1006 auser I/O in the user I/O queue. If the cache does not allow the user I/Oqueue, credit process 10 may fail 1008 the I/O request. For instance, inaddition to the I/O credits mechanism on the extent pool level, creditprocess 10 may also introduce disk I/O credits on the PDO level, whichmay be beneficial to have a two level credit system, since the I/Ocredits may only control the overall number of outstanding user I/Os.The user I/Os may break down to disk I/Os according to the RAID geometryand the RAID extent mapping. The traffic load of each disk may varygreatly, so before the extent pool credits are exhausted, some drivesmay already have been overloaded. In the example implementation, foreach I/O, credit process 10 may first take the I/O credits on the extentpool level, and then may also take the credits for each disk it touches(as discussed above). For any I/O, mapper RAID and the extent pool (viacredit process 10) may break it down to the disk I/Os. For each diskI/O, credit process 10 may take the credits from its associated disktouched. Only if all corresponding disks have enough credits, the I/Omay be processed, otherwise it may be rejected or pending.

However, in some implementations, if the extent pool does not haveenough credits (e.g., neither I/O credits nor disk credits), creditprocess 10 may have two additional choices. For instance, the DRAM Cache(via credit process 10) may set a flag on the user I/O to indicatewhether credit process 10 may queue this I/O while Mapped RAID hasinsufficient credits. If the DRAM Cache allows I/O queueing, creditprocess 10 may place 1006 the user I/O into the user I/O priority queue.If the DRAM Cache does not allow I/O queueing, credit process 10 mayfail 1008 the user I/O with an alter status to the DRAM Cache.

In some implementations, credit process 10 may return 1010 one or moreI/O credits used to process I/O 15 request based upon, at least in part,processing the I/O request. For example, once I/O 15 has been processedand completed, credit process 10 may return some or all of the disk I/Ocredits to the respective disks used to process I/O 15, by, e.g.,decreasing the corresponding disk I/O credits used to process I/O 15. Insome implementations, returning 1010 the I/O credits may includereturning some or all of the corresponding Ext pool I/O credits, by,e.g., decreasing the corresponding user I/O credits and/or backgroundI/O credits used to process I/O 15. In some implementations, the userI/O credits may be returned 1010 to the user I/O credits maintained onthe extent pool level, and the Background I/O credits may be return 1010to the Background I/O credits maintained on the extent pool level. Bothuser I/O and Background I/O may return disk I/O credits back tocorresponding disks (according to the I/O breaking down result). In someimplementations, credit process 10 may check if there are any user I/Ospending in the I/O priority queue, credit process 10 may wake up thepending I/Os in the priority queue (e.g., restart the I/O in the queue),and attempt to complete processing of that I/O, and return to the DRAMCache.

The terminology used herein is for the purpose of describing particularimplementations only and is not intended to be limiting of thedisclosure. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. As used herein, the language “at least one of A, B,and C” (and the like) should be interpreted as covering only A, only B,only C, or any combination of the three, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises” and/or “comprising,” when used in this specification,specify the presence of stated features, integers, steps (notnecessarily in a particular order), operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps (not necessarily in a particular order),operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents (e.g., ofall means or step plus function elements) that may be in the claimsbelow are intended to include any structure, material, or act forperforming the function in combination with other claimed elements asspecifically claimed. The description of the present disclosure has beenpresented for purposes of illustration and description, but is notintended to be exhaustive or limited to the disclosure in the formdisclosed. Many modifications, variations, substitutions, and anycombinations thereof will be apparent to those of ordinary skill in theart without departing from the scope and spirit of the disclosure. Theimplementation(s) were chosen and described in order to explain theprinciples of the disclosure and the practical application, and toenable others of ordinary skill in the art to understand the disclosurefor various implementation(s) with various modifications and/or anycombinations of implementation(s) as are suited to the particular usecontemplated.

Having thus described the disclosure of the present application indetail and by reference to implementation(s) thereof, it will beapparent that modifications, variations, and any combinations ofimplementation(s) (including any modifications, variations,substitutions, and combinations thereof) are possible without departingfrom the scope of the disclosure defined in the appended claims.

What is claimed is:
 1. A computer-implemented method comprising:receiving, by a computing device, an Input/Output (I/O) request fordata; determining a number of storage devices of a plurality of storagedevices in a Mapped Redundant Array of Independent Disks (RAID) groupthat will be used to process the I/O request, wherein determining thenumber of storage devices includes breaking down the received I/Orequest into a number of disk I/O credits via a Mapped RAID object anddetermining an amount of the number of disk I/O credits involved forprocessing the I/O request, wherein a capacity of space of the MappedRAID group is exposed to a cache through a Flare LUN (FLU) such thatwhen the I/O request is sent to the cache, a user I/O sends the I/Orequest to the FLU where the Mapped RAID group breaks the I/O requestinto a set of disk extents; determining that each storage device of thenumber of storage devices in the Mapped RAID group that will be used toprocess the I/O request lacks a respective threshold number of I/Ocredits to process the I/O request, wherein an I/O credit represents anI/O operation on a storage device of the plurality of storage devices inthe Mapped RAID group, wherein determining that each storage device ofthe number of storage devices in the Mapped RAID group lacks therespective threshold number of I/O credits includes calculating, for theuser I/O via an associated RAID object, which disk position I/O iscapable of at least one of accessing and requesting the number of diskI/O credits together with the I/O credit; determining whether the cacheassociated with the Mapped RAID group allows a user I/O queue inresponse to determining that each storage device of the number ofstorage devices in the Mapped RAID group that will be used to processthe I/O request lacks the respective threshold number of I/O credits; ifthe cache allows the user I/O queue, placing the user I/O in the userI/O queue; and if the cache does not allow the user I/O queue, failingthe I/O request.
 2. The computer-implemented method of claim 1 whereinan initial storage device credit for a first type of storage device ofthe plurality of storage devices is X.
 3. The computer-implementedmethod of claim 2 wherein the first type of storage device of theplurality of storage devices is a hard disk drive.
 4. Thecomputer-implemented method of claim 1 wherein an initial storage devicecredit for a second type of storage device of the plurality of storagedevices is Y.
 5. The computer-implemented method of claim 4 wherein thesecond type of storage device of the plurality of storage devices is aflash drive.
 6. The computer-implemented method of claim 1 wherein thethreshold number of I/O credits includes at least one of a number ofuser I/O credits in an extent pool and a number of background I/Ocredits in the extent pool.
 7. The computer-implemented method of claim1 further comprising returning the number of disk I/O credits used toprocess the I/O request based upon, at least in part, processing the I/Orequest.
 8. A computer program product residing on a non-transitorycomputer readable storage medium having a plurality of instructionsstored thereon which, when executed across one or more processors,causes at least a portion of the one or more processors to performoperations comprising: receiving an Input/Output (I/O) request for data;determining a number of storage devices of a plurality of storagedevices in a Mapped Redundant Array of Independent Disks (RAID) groupthat will be used to process the I/O request, wherein determining thenumber of storage devices includes breaking down the received I/Orequest into a number of disk I/O credits via a Mapped RAID object anddetermining an amount of the number of disk I/O credits involved forprocessing the I/O request, wherein a capacity of space of the MappedRAID group is exposed to a cache through a Flare LUN (FLU) such thatwhen the I/O request is sent to the cache, a user I/O sends the I/Orequest to the FLU where the Mapped RAID group breaks the I/O requestinto a set of disk extents; determining that each storage device of thenumber of storage devices in the Mapped RAID group that will be used toprocess the I/O request lacks a respective threshold number of I/Ocredits to process the I/O request, wherein an I/O credit represents anI/O operation on a storage device of the plurality of storage devices inthe Mapped RAID group, wherein determining that each storage device ofthe number of storage devices in the Mapped RAID group lacks therespective threshold number of I/O credits includes calculating, for theuser I/O via an associated RAID object, which disk position I/O iscapable of at least one of accessing and requesting the number of diskI/O credits together with the I/O credit; determining whether the cacheassociated with the Mapped RAID group allows a user I/O queue inresponse to determining that each storage device of the number ofstorage devices in the Mapped RAID group that will be used to processthe I/O request lacks the respective threshold number of I/O credits; ifthe cache allows the user I/O queue, placing the user I/O in the userI/O queue; and if the cache does not allow the user I/O queue, failingthe I/O request.
 9. The computer program product of claim 8 wherein aninitial storage device credit for a first type of storage device of theplurality of storage devices is X.
 10. The computer program product ofclaim 9 wherein the first type of storage device of the plurality ofstorage devices is a hard disk drive.
 11. The computer program productof claim 8 wherein an initial storage device credit for a second type ofstorage device of the plurality of storage devices is Y.
 12. Thecomputer program product of claim 11 wherein the second type of storagedevice of the plurality of storage devices is a flash drive.
 13. Thecomputer program product of claim 8 wherein the threshold number of I/Ocredits includes at least one of a number of user I/O credits in anextent pool and a number of background I/O credits in the extent pool.14. The computer program product of claim 8 wherein the operationsfurther comprise returning the number of disk I/O credits used toprocess the I/O request based upon, at least in part, processing the I/Orequest.
 15. A computing system including one or more processors and oneor more memories configured to perform operations comprising: receivingan Input/Output (I/O) request for data; determining a number of storagedevices of a plurality of storage devices in a Mapped Redundant Array ofIndependent Disks (RAID) group that will be used to process the I/Orequest, wherein determining the number of storage devices includesbreaking down the received I/O request into a number of disk I/O creditsvia a Mapped RAID object and determining an amount of the number of diskI/O credits involved for processing the I/O request, wherein a capacityof space of the Mapped RAID group is exposed to a cache through a FlareLUN (FLU) such that when the I/O request is sent to the cache, a userI/O sends the I/O request to the FLU where the Mapped RAID group breaksthe I/O request into a set of disk extents; determining that eachstorage device of the number of storage devices in the Mapped RAID groupthat will be used to process the I/O request lacks a respectivethreshold number of I/O credits to process the I/O request, wherein anI/O credit represents an I/O operation on a storage device of theplurality of storage devices in the Mapped RAID group, whereindetermining that each storage device of the number of storage devices inthe Mapped RAID group lacks the respective threshold number of I/Ocredits includes calculating, for the user I/O via an associated RAIDobject, which disk position I/O is capable of at least one of accessingand requesting the number of disk I/O credits together with the I/Ocredit; determining whether the cache associated with the Mapped RAIDgroup allows a user I/O queue in response to determining that eachstorage device of the number of storage devices in the Mapped RAID groupthat will be used to process the I/O request lacks the respectivethreshold number of I/O credits; if the cache allows the user I/O queue,placing the user I/O in the user I/O queue; and if the cache does notallow the user I/O queue, failing the I/O request.
 16. The computingsystem of claim 15 wherein an initial storage device credit for a firsttype of storage device of the plurality of storage devices is X.
 17. Thecomputing system of claim 16 wherein the first type of storage device ofthe plurality of storage devices is a hard disk drive.
 18. The computingsystem of claim 15 wherein an initial storage device credit for a secondtype of storage device of the plurality of storage devices is Y.
 19. Thecomputing system of claim 15 wherein the threshold number of I/O creditsincludes at least one of a number of user I/O credits in an extent pooland a number of background I/O credits in the extent pool.
 20. Thecomputing system of claim 15 wherein the operations further comprisereturning the number of disk I/O credits used to process the I/O requestbased upon, at least in part, processing the I/O request.